Oscillator circuits produce an output that varies periodically at a predetermined frequency. Oscillator circuits typically operate based on positive feedback. In a voltage-controlled oscillator (VCO), the principle tuning element is a varactor diode. The VCO is tuned across a frequency band by applying a DC voltage to the varactor diode, which varies the net capacitance of a tuned circuit.
VCOs may be implemented in frequency synthesizers such as phase-locked loops (PLLs). The PLLs, in turn, may be implemented in a device such as a wireless communications device. In wireless communications devices, the VCO provides a clock signal that is used by a transceiver during a frequency up-conversion and a down-conversion process. The clock signal ideally has no phase noise, which creates frequency fluctuations in the output signal. When the PLL is locked, the VCO may contribute noise at higher frequencies.
Referring to FIG. 1, an exemplary PLL 10 includes a phase detector 12, a charge pump 14, a filter 16, a VCO 18, and a divider 20. The phase detector 12 receives a reference frequency signal 22. For example, a crystal oscillator may be used to provide the reference frequency signal 22. The phase detector 12 also receives an output signal 24 from the divider 20. The phase detector 12 compares the reference frequency signal 22 and the divider output signal 24 and generates a phase error signal 26. The phase error signal 26 is a measure of the phase difference between the reference frequency signal 22 and the divider output signal 24. Typically, the phase error signal 26 is a DC voltage that is output to the charge pump 14, which converts the value of the phase error signal 26 into an absolute DC voltage or a VCO voltage signal 28.
Because the VCO 18 is sensitive to fluctuations in the VCO voltage signal 28, the filter 16 filters the VCO voltage signal 28 from the charge pump 14. The VCO 18 generates an output signal 32 at a desired frequency based on the value of the filtered VCO voltage signal 30.
The divider 20 receives the output of the VCO 32. Since the desired value for the output frequency of the VCO 18 is generally different than the reference frequency, the divider 20 is used to adjust the value of the output signal 32 based on the ratio of the desired output frequency to the reference frequency. Based on the feedback from the VCO 18, the PLL 10 locks the output signal 32 onto the reference frequency signal 22 and maintains a fixed relationship.
Referring now to FIG. 2, a first VCO circuit 40 includes first and second transistors 42 and 44, respectively. For example, the first and second transistors 42 and 44, respectively, may be bi-polar junction transistors (BJTs) that have bases, collectors, and emitters. However, other types of transistors may be used. An emitter (or terminal) of the first transistor 42 communicates with an emitter of the second transistor 44. The first VCO circuit 40 also includes first and second capacitors 46 and 48, respectively, that AC couple positive feedback of the first and second transistors 42 and 44, respectively. A first end of the first capacitor 46 communicates with a collector (or terminal) of the first transistor 42 and a second end of the first capacitor 46 communicates with a base (or control terminal) of the second transistor 44. A first end of the second capacitor 48 communicates with a collector of the second transistor 44 and a second end of the second capacitor 48 communicates with a base of the first transistor 42. The emitters of the first and second transistors 42 and 44, respectively, communicate with a first current source 50.
A net capacitance of the first VCO circuit 40 determines a frequency of an output signal. A control voltage signal 52 is applied to cathodes of first and second varactor diodes 54 and 56, respectively, to adjust the net capacitance of the first VCO circuit 40. For example, a charge pump may generate the control voltage signal 52 in a PLL. An anode of the first varactor diode 54 communicates with the collector of the first transistor 42, and an anode of the second varactor diode 56 communicates with the collector of the second transistor 44. A cathode of the first varactor diode 54 communicates with a cathode of the second varactor diode 56.
The first VCO circuit 40 that is illustrated in FIG. 2 is an LC-tank VCO that includes first and second inductors 58 and 60, respectively, at an output 62. A first end of the first inductor 58 communicates with the collector of the second transistor 44. A first end of the second inductor 60 communicates with the collector of the second transistor 44. The output signal 32 is referenced from second ends of the first and second inductors 58 and 60, respectively.
A DC signal may degrade when the VCO circuit includes coupling capacitors. Also, if the signal is directly coupled, the collector and base of the first and second transistors 42 and 44, respectively, will be at the same voltage. If the voltage is very low, this may drive the first and second transistors 42 and 44, respectively, into saturation. It is desirable to maintain the voltage at the collectors of the first and second transistors 42 and 44, respectively, relatively high with respect to the voltage at the bases of the first and second transistors 42 and 44, respectively. Therefore, a separate bias voltage signal 64 biases the bases of the first and second transistors 42 and 44, respectively through first and second resistors 66 and 68, respectively.
Beta values of the first and second transistors 42 and 44, respectively, may vary. This causes a difference in current through the first and second resistors 66 and 68, respectively, which leads to a difference in transconductance between the first and second transistors 42 and 44, respectively. Additionally, the bias voltage signal 64 is separately generated. The bias voltage signal 64 as well as the first and second resistors 66 and 68, respectively, contribute noise to the first VCO circuit 40.
Referring now to FIG. 3, a second VCO circuit 70 includes third and fourth transistors 72 and 74, respectively, that couple the positive feedback of the first and second transistors 42 and 44, respectively. For example, the third and fourth transistors 72 and 74, respectively, may be BJTs that operate as emitter followers, although other types of transistors may be used. An emitter of the third transistor 72 communicates with the base of the second transistor 44. A base of the third transistor 72 communicates with the collector of the first transistor 42. An emitter of the fourth transistor 74 communicates with the base of the first transistor 42. A base of the fourth transistor 74 communicates with the collector of the second transistor 44. A supply potential 76 is applied to collectors of the third and fourth transistors 72 and 74, respectively.
Current that flows through the third and fourth transistors 72 and 74, respectively, is sufficient to drive the bases of the first and second transistors 42 and 44, respectively. Therefore, the base of the third transistor 72 communicates with a second current source 78. The emitter of the third transistor 72 communicates with a third current source 80. The second VCO circuit 70 generates less noise than the first VCO circuit 40. The positive feedback of the second VCO circuit 70 does not have to be AC coupled. However, at high frequencies, the amount of current that the second VCO circuit 70 requires to reduce second harmonic distortion is very high.